JP3566899B2 - Method for producing polymer fine particles - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing polymer fine particles, and more particularly to a method for producing polymer fine particles having a plurality of bubbles therein.
[0002]
[Prior art]
Polymer fine particles have been used in various fields. Among them, the fine polymer particles having an average particle diameter of 1 to 100 μm appear white from the outside because light refracts the light when it hits the fine polymer particles. In particular, fine particles containing bubbles in the particles appear pure white due to complicated refraction of light. Therefore, such fine particles are useful as a white pigment. In particular, fine particles containing a plurality of bubbles in the particles have a whiter color more vivid than fine particles containing only one bubble. Therefore, polymer fine particles containing a plurality of bubbles and having an average particle diameter of 1 to 100 μm are useful as white pigments in the fields of paints, inks, cosmetics, and the like.
[0003]
JP-A-56-32513 describes a method for producing polymer particles containing bubbles therein. In this method, a core-shell latex having an alkali-swellable core is prepared, alkali-treated at a temperature equal to or higher than the glass transition point of the shell, and the shell is expanded by swelling the core. In this method, only the core is contracted, and a cavity is formed by contracting the core. In this method, since bubbles are generated after the core is shrunk, it is difficult to generate a plurality of bubbles in one particle, and particles having an average particle diameter of 1 to 100 μm are produced. It is difficult.
[0004]
JP-A-59-193901 discloses polymer particles containing one or more spherical bubbles in one particle by suspension polymerization of an aqueous phase / monomer phase / aqueous phase multiple emulsion. It describes how to make. In this method, it is necessary to add a large amount of each of the ionic surfactant and the nonionic surfactant to the aqueous phase and the monomer phase, respectively, which is not only costly but also troublesome to remove the surfactant. Therefore, fine particles containing bubbles cannot be efficiently produced.
[0005]
JP-A-5-125127 describes a method for producing polymer particles containing only one bubble in one particle. In this method, suspension polymerization is performed by coexisting a heterogeneous polymer in a monomer mixture composed of a hydrophilic monomer and a crosslinkable monomer. However, by using a special polymer that does not dissolve in the polymer formed by polymerization of the monomer mixture, polymer particles containing only one bubble are obtained. Therefore, this method cannot produce particles containing a plurality of bubbles.
[0006]
Further, polymer fine particles containing a plurality of bubbles can be produced by expanding the expandable particles, but it is difficult to reduce the average particle diameter of the expanded particles to about 1 to 100 μm.
[0007]
As described above, it has not been possible to efficiently produce polymer fine particles containing a plurality of bubbles in the particles.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for easily and efficiently producing polymer fine particles containing a plurality of bubbles therein by a simple operation.
[0009]
[Means for solving the problem]
The inventor of the present invention sought to include bubbles in the particles during the process of polymerizing the monomer. For that purpose, we tried to carry out suspension polymerization using various monomers. In the meantime, the present inventor conducted a suspension polymerization of a polymerizable solution obtained by dissolving a methacrylic acid ester polymer in a methacrylic acid ester monomer, and sometimes a polymer containing a large number of bubbles therein. It has been found that particles can be obtained.
[0010]
Based on this finding, the inventor further repeated experiments and found that when the methacrylic acid ester was a polymer obtained by an emulsion polymerization method, polymer particles containing a large number of bubbles were obtained. When suspension polymerization of a polymerizable solution obtained by dissolving a methacrylic acid ester polymer in a methacrylic acid ester monomer and maintaining the pH of the aqueous medium at 6 or more contains many foams. It has been found that the polymer particles are easily obtained. In addition, the present inventor has found that similar results can be obtained even when the above-mentioned monomer is not limited to a methacrylic acid ester but is an acrylic acid ester. Further, the inventor of the present invention has generally obtained a dispersion polymerization method, a precipitation polymerization method and a solution polymerization method using a water-soluble polymerization initiator instead of a polymer obtained by emulsion polymerization of (meth) acrylic ester. It has been found that similar results can be obtained using the obtained polymer. The present invention has been completed based on such findings.
[0011]
In the present invention, a (meth) acrylic ester is polymerized with a water-soluble polymerization initiator, and the obtained polymer is dissolved in the (meth) acrylic ester to form a polymerizable solution. Producing polymer fine particles containing a plurality of foams therein, wherein the polymerizable solution is dispersed and polymerized as fine particles of 1 to 100 μm in an aqueous medium maintained at a pH of 6 or more. It provides a method.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, (meth) acrylate is used as a monomer. That is, an acrylic ester and a methacrylic ester can be used. Acrylic ester and methacrylic ester both mean a monomer containing only one double bond in one molecule. The reason is that, for example, when a polymer having two double bonds in one molecule, such as vinyl acrylate, is used, the resulting polymer becomes a net-like polymer and cannot be dissolved in the monomer. is there.
[0013]
Acrylic esters that can be used in the present invention are, for example, alkyl acrylates such as methyl acrylate, ethyl acrylate, and n-butyl acrylate. The methacrylic esters that can be used in the present invention are, for example, alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, and n-butyl methacrylate. As these esters, it is preferable to use a single compound alone, but it is also possible to use a mixture of two or more compounds.
[0014]
In the present invention, when polymerizing the (meth) acrylic acid ester, other monomers can be copolymerized within a range that does not impair the effects of the present invention. Other monomers that can be copolymerized are, for example, chlorostyrene, vinyl acetate, (meth) acrylic acid, 2-hydroxyethyl (meth) acrylate, glycidyl (meth) acrylate, and the like.
[0015]
In the present invention, (meth) acrylate is first polymerized with a water-soluble polymerization initiator. As a medium for the polymerization, an organic solvent can also be used as long as it dissolves a water-soluble polymerization initiator. When water is used as the medium, it is preferable to use ion-exchanged water. The water-soluble polymerization initiator that can be used is generally called a radical polymerization initiator. Initiators which can be used include persulfates such as potassium persulfate, sodium persulfate and ammonium persulfate, and ions such as carboxyl groups in the molecule such as 4,4'-azobis-4-cyanovaleric acid and salts thereof. It is a water-soluble azo compound having a functional group.
[0016]
In the present invention, the method for polymerizing the (meth) acrylate is not limited to emulsion polymerization. For example, it is also possible to use a (meth) acrylate polymer obtained by dispersion polymerization of 4,4'-azobis-4-cyanovaleric acid using a mixture of methanol and water as a medium. it can. In addition, a polymer obtained by precipitation polymerization and solution polymerization using a water-soluble polymerization initiator can also be used. In the case of emulsion polymerization, a redox system may be used in the presence of a water-soluble reducing agent. As the water-soluble reducing agent, L-ascorbic acid, sodium pyrosulfite and the like can be used. In carrying out the emulsion polymerization, it is preferable to use a chain transfer agent. As the chain transfer agent, for example, mercaptans such as n-octyl mercaptan and α-methylstyrene dimer can be used.
[0017]
In the present invention, it is preferable not to use a surfactant when polymerizing the (meth) acrylate with a water-soluble polymerization initiator. The reason is that when a surfactant is used, the obtained polymer has poor water resistance, but when polymerized without using a surfactant, the obtained polymer has water resistance. Because.
[0018]
In the present invention, it is preferable that the (meth) acrylate polymer obtained as described above has a weight average molecular weight of about 10,000 to 300,000. The reason is that a polymer having such a weight average molecular weight is easily dissolved in a (meth) acrylate. The polymer having the above-mentioned weight average molecular weight can be easily obtained by using the above-mentioned chain transfer agent.
[0019]
In the present invention, the above (meth) acrylate polymer is dissolved in the (meth) acrylate to form a polymerizable solution. In this case, the (meth) acrylate serving as the solvent is preferably the same compound as the (meth) acrylate constituting the polymer. For example, if the constituent of the polymer is methyl methacrylate, it is preferable to use methyl methacrylate as a solvent to form a polymerizable solution. The reason is that, when the constituents of the polymer and the constituents of the solvent match, the polymer is easily dissolved in the solvent.
[0020]
When the above polymer is dissolved in a solvent to form a polymerizable solution, the mixing ratio of the two is not particularly limited. As long as the (meth) acrylate serving as a solvent dissolves the polymer to form a solution, and as long as the solution can be dispersed as fine particles in an aqueous medium, both can be used in combination. Usually, the amount of the polymer can be used within the range of 0.1 to 100 parts by weight based on 100 parts by weight of the (meth) acrylic acid ester which becomes a solvent.
[0021]
In the present invention, the polymerizable solution obtained as described above is dispersed as fine particles in an aqueous medium. In this case, it is preferable to use ion-exchanged water as the aqueous medium. The aqueous medium in this case preferably contains a dispersion stabilizer. Water-soluble polymer compounds such as methylcellulose, hydroxyethylcellulose and polyvinyl alcohol are used as dispersion stabilizers, and inorganic salts which are hardly soluble in water such as tertiary calcium phosphate, hydroxyapatite, magnesium hydroxide and magnesium pyrophosphate are used. be able to.
[0022]
The above-mentioned inorganic salts can be used by dispersing a commercially available fine powder in water as it is, for example, by reacting tribasic sodium phosphate and calcium chloride in water and subjecting the tribasic calcium phosphate to metathesis to form the tribasic calcium phosphate. It can also be generated and used. It is preferable that the amount is 0.1 to 20 parts by weight per 100 parts by weight of the polymerizable solution. The inorganic salts can be easily removed from the polymer particles by washing the polymer particles obtained by polymerization with water or treating with hydrochloric acid. Further, inorganic salts are effective in narrowing the particle size distribution of the polymer particles. Therefore, it is preferable to use inorganic salts as the dispersion stabilizer.
[0023]
On the other hand, when the above-mentioned water-soluble polymer compound is used, it strongly adheres to the surface of the polymer particles obtained by polymerization, so that it is not easy to remove it. When the polymer particles to which this is attached are heated, the attached substance causes the polymer particles to be colored. Therefore, it is preferable to avoid using a water-soluble polymer compound as much as possible.
[0024]
The aqueous medium described above may contain a surfactant in addition to the dispersion stabilizer. It is preferable to use an anionic surfactant. Suitable surfactants are sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, sodium polyoxyethylene lauryl ether sulfate, sodium dioctyl sulfosuccinate, and the like. Although the amount of use is not particularly limited, it is usually used at a ratio of 0.005 to 0.3% by weight based on the aqueous medium.
[0025]
In the present invention, the above-mentioned polymerizable solution is subjected to suspension polymerization in an aqueous medium. For that purpose, it is necessary to add a polymerization initiator to the polymerizable solution and dissolve the polymerization initiator in the polymerizable solution. As the initiator that can be dissolved in the polymerizable solution, those known as a polymerization initiator for suspension polymerization can be used. Examples include azo compounds such as 2,2'-azobis (2,4-dimethylvaleronitrile) and azoisobisbutyronitrile, dilauroyl peroxide, dibenzoyl peroxide, and t-butylperoxy-2-. Organic peroxides such as ethylhexanoate. These initiators can be used alone or in combination of two or more. Although the amount of the initiator used is not particularly limited, it is usually used in a proportion of 0.01 to 3% by weight based on the monomer (meth) acrylate.
[0026]
In the present invention, the above-mentioned polymerizable solution, particularly a polymerizable solution containing a polymerization initiator, is dispersed as fine particles of 1 to 100 μm in an aqueous dispersion medium. This dispersion can be easily achieved by using a disperser such as a homogenizer, in particular, by adjusting the shearing force by changing the rotation speed of the disperser.
[0027]
After the polymerizable solution is finely dispersed in the aqueous medium in this manner, the pH of the aqueous medium is maintained at 6 or more while stirring is continued, and the aqueous medium is heated to perform suspension polymerization. The reason for setting the pH to 6 or more is based on the results of experiments. That is, it has been confirmed by experiments that the polymer particles containing bubbles cannot be obtained when the pH is less than 6. When the pH is 6 or more, the pH is preferably 7 or more. Conversely, if the pH exceeds 11, the dispersion of the polymerizable solution tends to be unstable, and the (meth) acrylate ester is easily hydrolyzed. Therefore, the pH is preferably 11 or less.
[0028]
To maintain the pH of the aqueous medium at 6 or higher, the pH is adjusted to 6 or higher at the beginning of the polymerization by selecting a dispersion stabilizer or by adding an acid or a base. An inorganic acid such as hydrochloric acid or an organic acid such as citric acid can be used as the acid, and sodium or potassium hydroxide can be used as the base. Further, during the polymerization, the pH of the aqueous medium may change due to decomposition of the polymerization initiator or the like. Therefore, if necessary, the pH is maintained at 6 or more by adding the above-mentioned acid or base.
[0029]
In the present invention, the polymerizable solution is subjected to suspension polymerization by heating the aqueous medium and maintaining the aqueous medium at an appropriate temperature while maintaining the pH of the aqueous medium at 6 or more. Then, the polymerizable solution is polymerized into a (meth) acrylate polymer. After the polymerization, if necessary, the dispersant is decomposed with an acid or the like, and the polymer is filtered, washed, and dried. The polymer fine particles thus obtained are pulverized and classified as necessary to obtain fine particles having a particle diameter of 1 to 100 μm.
[0030]
The polymer particles thus obtained contain a plurality of bubbles inside. For this reason, the polymer fine particles appear pure white and are very beautiful. Therefore, the polymer fine particles are suitable as a white pigment as an additive for paints, inks, and cosmetics.
[0031]
It is not clear why the polymer microparticles obtained by the present invention contain a plurality of bubbles. However, the present inventors presume that the polymer of (meth) acrylate obtained by polymerization with a water-soluble polymerization initiator has a decomposed fragment of the polymerization initiator bonded to the end of the chain molecule. It is considered that the solution obtained by dissolving the polymer in the monomer for this purpose, when dispersed in an aqueous medium having a predetermined pH, particles containing the aqueous medium as water droplets. It is easy to produce, and since the particles are stable and polymerize as they are, polymer particles containing a plurality of water droplets are obtained, and this is dried later, so that the water droplets evaporate and foam is generated later Seems to be.
[0032]
【The invention's effect】
According to the present invention, (meth) acrylic acid ester is first polymerized with a water-soluble polymerization initiator to form a polymer, and then the polymer is dissolved in (meth) acrylic acid ester to form a polymerizable solution. This polymerizable solution is dispersed as fine particles of 1 to 100 μm in an aqueous medium, and the polymerizable solution is subjected to suspension polymerization in an aqueous medium having a pH of 6 or more, whereby a polymer containing a plurality of bubbles is obtained. Fine particles can be obtained. Therefore, according to this method, it is possible to obtain polymer fine particles containing a plurality of bubbles only by performing the polymerization with the water-soluble polymerization initiator and the suspension polymerization once, respectively. It is possible to efficiently obtain fine particles containing therein. The fine particles thus obtained are useful as additives for paints, inks and cosmetics, and as adhesives and resin modifiers. In this respect, the present invention has a remarkable effect.
[0033]
Hereinafter, the effects of the present invention will be specifically described with reference to Examples and Comparative Examples.
[0034]
Embodiment 1
-Emulsion polymerization-
A separable flask equipped with a stirrer, a thermometer and a reflux condenser is charged with 600 g of water, 100 g of methyl methacrylate, and 0.5 g of n-octyl mercaptan (n-OM) as a chain transfer agent. The temperature was raised. The internal temperature was maintained at 70 ° C., and 0.5 g of potassium persulfate was added as a water-soluble polymerization initiator, followed by a reaction for 8 hours. From the obtained emulsion, a polymer was taken out by salting out, washed with water and dried. The obtained poly (methyl methacrylate) had a weight average molecular weight of 42,000.
[0035]
-Suspension polymerization of polymerizable solution-
After 500 g of ion-exchanged water, 10 g of tribasic calcium phosphate and 0.12 g of sodium dodecyl sulfate were mixed in a 2 L beaker, the pH value was adjusted to 9.0 with sodium hydroxide to obtain an aqueous dispersion medium. Next, 10 g of the above-mentioned polymethyl methacrylate obtained by carrying out emulsion polymerization and a polymerizable composition obtained by dissolving 1 g of azobisisobutyronitrile as a polymerization initiator in 90 g of methyl methacrylate were charged, and finely divided using a homomixer. A suspension was formed by dispersing the mixture into droplets. The above suspension was charged into a separable flask equipped with a stirrer, a thermometer and a reflux condenser, and the temperature was raised to 60 ° C. while purging with nitrogen while stirring. Further, the internal temperature was maintained at 60 ° C., and the reaction was performed for 12 hours.
[0036]
After completion of the reaction, tribasic calcium phosphate was dissolved with hydrochloric acid, filtered and removed, washed with water and dried to obtain polymer fine particles. Observation of the obtained polymer fine particles using a magnifying glass with a magnification of 200 times revealed that the particles had a large number of vacuoles inside, and the average particle diameter was 18 μm according to the Coulter method. The obtained reflectivity was 25.5%.
[0037]
For comparison, a light scattering property was measured using a commercially available polymer fine particle (Matsumoto Microsphere M-600, manufactured by Matsumoto Yushi Seiyaku Co., Ltd.) containing only one bubble in the particle. The reflectance was 24.0%, and the light scattering was clearly inferior to the polymer fine particles of Example 1.
[0038]
The light scattering property was measured as follows. A coating composition is prepared by sufficiently mixing 3 g of polymer fine particles and 10 g of a polyester-based binder (manufactured by Toyobo Co., Ltd., trade name: Vironal MD1200, solid content: 34%) and uniformly dispersing the fine particles in the binder. The obtained composition is applied to a black opacity test paper using an applicator having a wet thickness of 100 μm and dried in an oven at 60 ° C. to obtain a sample piece on which a fine particle-containing coating film is formed. Using a colorimeter (manufactured by Minolta, product name: CR-300), the reflectance of the sample was measured using an XYZ color system. The larger the value, the greater the light emitted by the polymer particles contained in the coating layer. It was evaluated that scattering was large.
[0039]
[Comparative Example 1]
-Implementation of suspension polymerization instead of emulsion polymerization-
In a separable flask equipped with a stirrer, thermometer and reflux condenser, 600 g of water, 10 g of tribasic calcium phosphate, 0.1 g of sodium lauryl sulfate, 100 g of methyl methacrylate, and 0.1 g of azoisobisbutyronitrile, which is poorly water-soluble as a polymerization initiator. 5 g was charged, the temperature was raised to 60 ° C. while purging with nitrogen under stirring, and the internal temperature was kept at 60 ° C., and the mixture was reacted for 15 hours to carry out suspension polymerization. After the completion of the polymerization reaction, tribasic calcium phosphate was removed using hydrochloric acid, followed by washing with water and drying to obtain polymer fine particles. The obtained polymer had a weight average molecular weight of 800,000.
[0040]
-Suspension polymerization of polymerizable solution-
Using the above polymer, suspension polymerization of a polymerizable solution was performed in exactly the same manner as in Example 1. Observation of the obtained polymer fine particles using a magnifying glass having a magnification of 200 times revealed that the obtained polymer fine particles were true spherical particles having no vacuoles therein, and had an average particle diameter of 9 μm according to the Coulter method.
[0041]
[Comparative Example 2]
In this comparative example, polymethyl methacrylate obtained by performing emulsion polymerization in Example 1 was used, and this was dissolved in methyl methacrylate to form a polymerizable solution. This polymerizable solution was prepared in the same manner as in Example 1. The suspension polymerization was carried out in the same manner as in Example 1 except that the pH value of the aqueous system was adjusted to 5.5 using citric acid.
[0042]
Observation of the obtained polymer fine particles using a magnifying glass with a magnification of 200 times revealed that the obtained polymer fine particles were spherical particles having no vacuoles inside, and had an average particle diameter of 15 μm according to the Coulter method.
[0043]
Embodiment 2
-Emulsion polymerization-
A poly (n-butyl methacrylate) having a weight average molecular weight of 56,000 polymerized by performing an emulsion polymerization method in the same manner as in Example 1 except that n-butyl methacrylate was used instead of methyl methacrylate in Example 1. Got.
[0044]
-Suspension polymerization of polymerizable solution-
Same as Example 1 except that the above poly-n-butyl methacrylate and n-butyl methacrylate were used instead of methyl methacrylate in Example 1, and that magnesium pyrophosphate was used instead of tertiary calcium phosphate. To perform polymerization. After the completion of the reaction, the metathesis magnesium pyrophosphate was dissolved and removed using hydrochloric acid, washed with water and dried to obtain polymer fine particles.
[0045]
Observation of the obtained polymer fine particles using a magnifying glass with a magnification of 200 times revealed that the polymer fine particles had a large number of vacuoles inside, and had an average particle diameter of 10 μm as measured by the Coulter method.
[Brief description of the drawings]
FIG. 1 is an enlarged cross-sectional view of a polymer fine particle obtained by the present invention.
FIG. 2 is a photomicrograph of a cross section of a polymer fine particle obtained by the present invention.

Claims (2)

The (meth) acrylic ester is polymerized with a water-soluble polymerization initiator, and the obtained polymer is dissolved in the (meth) acrylic ester to form a polymerizable solution, and the polymerizable solution is 1 to 100 μm in an aqueous medium. A method for producing polymer fine particles containing a plurality of bubbles inside, wherein the polymerizable solution is dispersed and polymerized in an aqueous medium maintained at a pH of 6 or more. It is characterized by using persulfate or a water-soluble azo compound containing an ionic group in the molecule as a water-soluble polymerization initiator, and polymerizing (meth) acrylate without using a surfactant. The method of claim 1.

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